As a technique to improve an output of an engine, a method (supercharging) of compressing intake air with a turbocharger and supplying an engine with the compressed intake air is known, and widely used in engines for automobiles and the like. Especially in recent years, supercharging is applied to a wide operation range of from a low-output range to a high-output range, in view of the regulations on exhaust gas and fuel consumption rate that have become increasingly stricter.
On the other hand, in a turbocharger, surging may occur transitionally depending on the operation state of the turbocharger. Occurrence of surging in a turbocharger may lead to breakage of various devices such as an impeller, and thus needs to be prevented as much as possible. In view of this, a technique to widen the operation range of a compressor while avoiding occurrence of surging has been required.
A known technique to widen the operation range of a compressor while avoiding occurrence of surging is an intake bypass device which guides a part of compressed intake air compressed by a compressor to flow to an upstream side of a turbine bypassing an engine body, as disclosed in Patent Document 1, for instance.
The intake bypass device of Patent Document 1 includes a bypass channel connecting a downstream side of the compressor in an intake channel and an upstream side of the turbine in an exhaust channel, and a flow-rate adjustment valve disposed in the bypass channel. The flow-rate adjustment valve is controlled so that the valve opening degree increases when the operation point of the compressor enters the vicinity of a surge region. When the valve opening degree of the flow-rate adjustment valve increases, the flow rate of the compressed intake air guided to flow to the upstream side of the turbine via the bypass channel increases. As a result, the turbine output increases and the operation flow rate of the compressor increases, which makes it possible to prevent surging.